The present invention relates to a fixing roller temperature control in an image forming apparatus for heating and fixing a toner image transferred on a recording medium, by means of a fixing roller.
Generally in an image forming apparatus based on an electrophotographic system, extensive use is made of a fixing apparatus provided with a fixing heater roller in contact with one side of an image support and a pressure roller arranged so as to be clamped on the fixing heater roller, so as to ensure that the toner image transferred on one side of the image support (recording medium), such as transfer paper, is thermally fixed on the image support.
In some fixing apparatuses, a heat lamp such as a halogen lamp (hereinafter referred to as “heater”) or a heating source based on an induction heating method (heating means) is used to heat the fixing heater roller.
In such a fixing apparatus, a target fixing temperature is preset, and power supply to the aforementioned heating means is controlled so that the temperature of the fixing roller will reach the target fixing temperature. To provide such temperature control, a temperature sensor is arranged close to the fixing roller to detect the temperature of the fixing roller. The temperature sensor has a thermal response characteristic called “responsivity”, and has a problem in that it gets the result of measurement after the lapse of a predetermined time interval since it is incapable of providing an instant measurement of the temperature of a subject.
In view of the aforementioned problem of responsivity, the rate of change in the detected temperature, the responsivity (which is a time constant) of the temperature sensor, and the correction rate are added to provide the temperature control of the fixing roller, according to Patent Document 1 identified below.
In Patent Document 2 identified below, the sampling time of the temperature sensor is set to a shorter level when paper is fed, than when paper is not fed, thereby improving temperature control precision.
[Patent Document 1] Official Gazette of Japanese Patent Tokkaihei 5-258761
[Patent Document 2] Official Gazette of Japanese Patent Tokkai 2002-148994
In the temperature control described in Patent Document 1, overshoot can be reduced by correcting the temperature if there is a rise of fixing roller temperature. However, the same correction is also applied when the temperature of the fixing roller falls. Accordingly, an undershoot is produced conversely by the delay of response due to the responsivity of the temperature sensor. Thus, the temperature ripple (the difference between the maximum and minimum values) in temperature control cannot be reduced in total. This has been a problem of this prior art.
In the temperature control described in the aforementioned Patent Document 2, the temperature ripple can be reduced to some extent when the sampling time is set to a shorter level. However, the problem cannot be solved regarding the delay in response due to the responsivity of the temperature sensor. Thus, a great temperature ripple remains without being reduced.
In recent years, an image forming apparatus using the heating means based on the induction heating (IH) system has made its debut on the market. Even if the maximum power of the fixing section is 1000 watts, this image forming apparatus allows the 1000-watt power to be used on either the end or center of the fixed roller. When only the end or the center of the fixed roller is taken into account, the image forming apparatus of this type has a rate of temperature rise equivalent to double the rate of temperature rise of a conventional product. Thus, the impact of the ripple tends to increase.